1. Field of the Invention
This invention relates to plastically deformable aqueous ceramic slurries and methods of making such compositions. In particular, this invention relates to making aqueous ceramic slurries or mixtures of oxide and nonoxide ceramic particles that demonstrate a high degree of plasticity to enable such compositions to be formed or shaped into useful articles or structures having intricate shapes by conventional plastic deformation processes, e.g., injection molding and extrusion.
2. Prior Art
Plastic-forming methods, such as injection molding and extrusion, are essential to fabricating ceramic objects with intricate shapes. While known mixtures of clay and water are plastic and can be easily formed into useful shapes, aqueous slurries of advanced ceramics do not show such plasticity.
Typically, to obtain a plastically deformable ceramic composition of nonplastic ceramic powders dispersed in a carrier medium, binders are added to the composition. For example, Mutsuddy, xe2x80x9cInjection Molding,xe2x80x9d pp. 173-80 in Engineered Materials Handbook Vol.4, Ceramics and Glasses, ASM Int., 1991, describes making such plastically deformable ceramic compositions by adding clay, a polymer solution, and thermal plastics to the ceramic composition. There are several problems associated with such compositions. In particular, adding clay into the formulation changes the final composition, thus limiting the applications for the ceramic materials, the removal of the polymeric binders tends to induce defects in the ceramic product that is produced and the use of such polymeric binders can create health and environmental risks.
Berstrxc3x6m et al., xe2x80x9cConsolidation Behavior of Flocculated Alumina Suspensions,xe2x80x9d J. Am. Ceram. Soc., 75 [12] 3305-14 (1992) attempted to overcome these problems by endowing plasticity to ceramic slurries through the control of the interparticle forces. This was done by modifying the surface of the alumina powders by adsorbing a monolayer of fatty acid in decaline. Bergstrxc3x6m et al. demonstrated that when the chain length of the adsorbed fatty acid reached a critical value, the resulting interparticle forces consisted of weakly attractive forces that became strongly repulsive forces at shorter interparticle separations. The suspensions produced by Bergstrxc3x6m et al. could be packed to high density (xcx9c60 vol % solids loading) by centrifugation and the cakes produced by such centrifugation were plastic and extrudable. See also, Schilling, et. al., xe2x80x9cParticle Attraction Effects on the Centrifugal Casting and Extrusion of Aluminaxe2x80x9d; pp. 35-51 in Handbook on Characterization Techniques for the Solid-Solution Interface. Edited by J. H. Adair, J. A. Casey, and S. Venigalla. Am. Ceram. Soc., Westerville, Ohio, 1993. However, although these formulations had certain advantages over, for example, the compositions of Mutsuddy, they can only be formulated with nonpolar organic solvents, which posehealth and environmental risks.
In order to avoid such risks, attempts have been made to induce plasticity to aqueous slurries of advanced ceramics by using xe2x80x9chydration forcesxe2x80x9d or a lipid bilayer. Velamakanni, et al., xe2x80x9cNew Method for Efficient Colloidal Particle Packing via Modulation of Repulsive Lubricating Hydration Forcesxe2x80x9d, Langmuir 6 1323-25 (1990). E. P. Luther, et. al., xe2x80x9cThe Development of Short-Range Repulsive Potentials by Short-Chain Surfactants in Aqueous Si3N4 Slurriesxe2x80x9d, submitted to the Journal of the American Ceramic Society. However, the consolidated slurries were either nonextrudable Schilling, et. al. or fluid-like Luther, et. al.
Additionally, it is known that ceramic particles can be dispersed in water by the adsorption on the particles of an ionic/nonionic surfactant bilayer. Somasundaran et al., xe2x80x9cCoadsorption of anionic and nonionic surfactant mixtures at the alumina-water interface.,xe2x80x9d Langmuir 8 [4] 1065-69 (1992); Fu et al, xe2x80x9cThermodynamic Study of Adsorption of Anionic-Nonionic Surfactant Mixture at the AluminaWater Interface,xe2x80x9d pp. 366-76 in Mixed Surfactant Systems, ACS symp. Series 501. Edited by P. M. Holland and D. N. Rubingh, Amer. Chem. Soc., Washington, D.C., 1992; and Esumi et al, xe2x80x9cMixed bilayers of anionic and nonionic surfactants on alumina.,xe2x80x9d J. Colloid Interface Sci. 134 [1] 283-88 (1990).
It is an object of this invention to provide a plastically deformable aqueous ceramic slurry.
It is a further object of this invention to provide aqueous slurries of ceramics that can be molded into intricate shapes by injection molding or extrusion.
It is yet another object of this invention to provide ceramic slurries that can be molded into intricate shapes by injection molding or extrusion, but do not use polymeric binders, thus reducing defects in the final ceramic product and reducing health and environmental risks caused by the use of such binders.
It is still another object of this invention to provide such ceramic slurries without using nonpolar organic solvents, thereby reducing the health and environmental risks caused by the use of such solvents.
All of the foregoing objects are obtained by the plastically deformable aqueous ceramic slurries of this invention wherein the ceramic particles have on the surfaces thereof a closely-packed anionic surfactant bilayer oran ionic/nonionic surfactant bilayer. Optionally, such ceramic particles have on the surfaces thereof a closely-packed cationic surfactant bilayer or a cationic/nonionic surfactant bilayer. Such slurries are produced by:
a) dispersing an amount of ceramic particles in an amount of water to form an aqueous ceramic slurry;
b) adding an amount of an anionic surfactant or a mixture of an anionic surfactant and nonionic surfactant to such slurry;
c) adjusting the pH value of such slurry to adsorb on the ceramic particle surfaces a closely-packed anionic surfactant bilayer or anionic/nonionic surfactant bilayer.
Optionally, a cationic surfactant may be used instead of an anionic surfactant. Preferably a mixture of surfactants is used, with a mixture of anionic and nonionic surfactants being highly preferred. Examples of preferred combinations are mixtures of the anionic surfactant (SDS) and a nonionic surfactant selected from octylphenol polyether alcohols (OPE), in particular C8H15Ph(EO)Rxe2x80x94 wherein R was 3, 5, 9.5 and 12.5.
Both the pH value and surfactants are selected so that the formed bilayer is tenaciously adsorbed onto the ceramic particle surface.
The dispersed or weakly flocculated ceramic suspensions can then be packed by high density centrifugation or pressure filtration to form consolidated ceramic cakes which are plastic and extrudable allowing them to be shaped into complicated structures by various plastic forming methods.
The compositions and processes of this invention are advantageous in that water is used instead of organic solvents, thus minimizing environmental and health hazards, the total amount of organic additives used is generally less than 1 wt % of the ceramic slurry, thus reducing costs and avoiding problems associated with binder burnout; and the surfaces of the ceramic particles are coated with hydrophobic layers, allowing aqueous colloidal processing of water-sensitive ceramic powders.